Tin(II) (stannous) ions, provided in oral compositions by stannous fluoride and/or other stannous salts, have long been valued for the multiple benefits that they can afford, including antimicrobial effects, control of breath malodor, control of dental plaque growth and metabolism, reduced gingivitis, decreased progression to periodontal disease, reductions in dentinal hypersensitivity, and reduced coronal and root dental caries and erosion. Along with the benefits however there are some notorious problems. One of the most notable side effects of regular use of stannous fluoride is yellow-brown tooth staining. This stain is derived from pellicle, plaque and dietary component reactions with available stannous deposited on tooth surfaces during treatment with effective stannous fluoride formulations. A second side effect routinely encountered during use of effective stannous fluoride formulations is unacceptable formulation astringency. Furthermore, formulating stannous ions stably also presents a challenge as the tin(II) ion is both prone to oxidation towards tin(IV) and to precipitate from aqueous solution as stannous hydroxide. The latter is a pH dependent phenomenon and is typically avoided by formulating at a low pH and/or by formulating in an anhydrous composition. Formulating at a low pH is not preferred when a fluoride source is utilised in the presence of a silica dental abrasive however because it increases the tendency of the fluoride to react with the silica. Formulating in aqueous compositions though is of advantage for reasons, such as cost and formulation flexibility. Another approach to stabilising stannous is to include a chelating agent in the composition as disclosed e.g., in U.S. Pat. No. 3,282,792, WO 96/17587, U.S. Pat. Nos. 5,004,597, 5,213,790 and US 2007/0025928.
Zinc ions are also advantageously included in oral compositions. Combining zinc ions with stannous ions can give a broader spectrum of anti-microbial activity but zinc ions also pose increased formulation challenges through competing with the chelating agents used to stabilise stannous. The astringency of zinc is also well known and it has been found that the degree of astringency depends upon the form in which zinc is present in the composition. EP 426 213 discloses oral compositions comprising the combination of zinc and stannous as an anti-plaque system.
WO 94/14406 and WO 94/14407 describe formulating a source of zinc ions, preferably zinc oxide or zinc nitrate, along with sources of citrate and pyrophosphate ions in defined ratios. WO 00/61092 discloses increasing the bioavailability of zinc in a dentifrice by buffering the dentifrice at a pH of from 3 to 5.5. Its zinc is provided by slightly soluble zinc compounds, especially zinc citrate.
WO 2007/076001 discloses oral care compositions comprising an essentially water-insoluble zinc compound and phytate. Some of its example compositions include stannous ions.
Despite all of the foregoing, further improvements are needed in the formulation of zinc into oral compositions, in order to deliver the combination of anti-plaque efficacy with acceptable taste.
It has now been found that, by careful choice of chelant levels, oral compositions comprising fluoride, zinc and stannous can be formulated at a higher pH than has typically been used, without compromising on stannous stability. This provides advantages in fluoride stability and has also been found to avoid taste problems from the zinc.